Torque limiting device

ABSTRACT

A work string assembly for an oil or gas well has a first and second tubulars, and a torque limiting device connected in the work string. The torque limiting device has a body with an axial bore and two interconnected members that can disconnect to be relatively rotatable. The interconnected members can be repeatedly re-engageable when the torque levels drop below a torque threshold value, or can be connected by a frangible member that is broken when the interconnected members disconnect. A selectively actuable circulation port provided in the string adjacent to and above the torque limiting device allows the annulus between the work string and the casing to be flushed with fluid from the port.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to downhole tools and particularly, thoughnot exclusively, to a torque limiting device used to allow relativerotation between two tubulars when a predetermined torque is appliedbetween the tubulars, thereby reducing or preventing torsional damage tothe tubulars or the connection between them at torque loads above thethreshold of the predetermined torque.

2. Description of the Related Art

Wellbores typically contain various strings of tubulars connected end toend in the well. From time to time, for example, after drilling andcasing a well, the inner surface of a tubular string such as a casingstring is conveniently cleaned with a cleaning string to remove rust andother debris from the well bore. When a cleaning string is rotated bythe powerful rig top drive to scrub the inner surface of the casingstring, an operator must be careful to avoid applying too much torque tothe cleaning string because it is common for high torque levels that canbe applied by the top drive to damage the cleaning string, requiringcostly intervention to repair or recover the damaged string.

The actual torque applied to each part of the string can be affected bythe properties of the work string itself and by the properties of fluidthat locally surround that part. Additional problems can arise in wellbore cleaning operations because of the practice of circulatingdifferent cleaning fluids such as brine through the string at the bottomof the well in order to displace drilling mud or other lubricant fromthe annular space between the casing string and the cleaning string. Asa result, different parts of the string can be surrounded locally bydifferent fluids, and as a result, the two parts can be exposed todifferent frictional forces that resist the string's rotation todifferent extents. For example, strings rotating in brine haveapproximately twice the drag as compared with those run in mud and themethod of pumping the brine down through the inner work string outthrough an aperture at the bottom of the work string and then up theannulus outside the work string to displace the mud ahead of it meansthat the lowermost section of a work string that is being used to pumpbrine may be rotating in brine whereas the uppermost might be rotatingin low friction drilling mud. Such a string will therefore experiencedifferential drag along its length. Accordingly, the torque experiencedby the lower section of the work string can be different from the torqueexperienced at the upper section. The moving interface between the brineand the drilling mud as the mud is displaced upwards along the annulusresults in additional complications for the operator, and the risks ofexceeding the mechanical properties of the lower string are increased asthere is no mechanism to monitor from the surface the torque that isexperienced by the lower section of cleaning string in these variableconditions.

Furthermore tubular strings are often arranged in sequential sections ofgradually decreasing diameter, and adjacent sections of tubulartypically create steps in the diameter of the string, for example, in awork string, an upper section can often have a larger outer diameterthan a lower section. Also, a casing string can have steps in the innerdiameter, for example where a liner is hung inside casing. Difficultiesarise in satisfactory removal of debris from e.g. casing strings with anon-uniform inner diameter because there is frequently a large annularspace between some portions of the work string bearing cleaning toolsand the upper sections of the casing string being cleaned. Cleaningstrings used to carry out this cleaning task are therefore typicallymade up with at least two sequential sections of decreasing diameter sothat the larger diameter upper sections of casing string are cleaned bya larger diameter section of the work string. Accordingly the cleaningstring can typically have at least one step (or crossover) in itsdiameter and the relative mechanical properties of upper and lowersections of the cleaning string can therefore be very different. Thisincreases the risks of torsional damage to the crossover or the weakernarrow diameter pipe below it.

In a well with 4 to 5,000 feet of liner, when the brine is circulated tothe top of the liner, the rotating tubular string will experience anincrease in torque as described above. To avoid damage to the lowerstring typically a circulation tool with a clutch device is located inthe string and positioned at the liner top. The clutch device allows thelower string to be selectively mechanically disengaged from the upperstring. While the clutch is engaged, rotation of the upper string istransmitted across the clutch and the upper and lower strings rotatetogether, but when the clutch is disengaged, the upper string can freelyrotate while the lower string remains stationary. The circulation tooland clutch tend to be interdependent or even incorporated into a singletool. When the circulating brine reaches the liner top, the clutch isengaged and the circulation tool is activated to increase the pump rateof brine into the wider casing. However, when the circulation tool isready to be activated and the clutch engaged, the relatively narrowliner annulus is already full of brine and maximum torque is alreadyapplied to the narrow section of the cleaning string below thecirculation tool. Currently, in an attempt to prevent damage to thestring from the torque, an operator will stop rotation of the stringearly. This compromises cleaning of the liner as mud may be left at theliner top. Alternatively if rotation is not stopped sufficiently earlythe lower string and/or the crossover may be damaged by the torque.

SUMMARY OF THE INVENTION

The invention provides a torque limiting apparatus for a work string ofan oil or gas well, the torque limiting device comprising a body havingan axial bore therethrough for fluid flow; the body comprising a firstmember and a second member interconnected to be relatively rotatable;the first and second members being interconnected by a torque limitingdevice adapted to transfer torque between the first and second membersup to a threshold torque value, thereby rotationally coupling the firstand second members together at torque levels below the threshold torquevalue, and wherein the torque limiting device is adapted to disengagethe connection between the first and second members when torque appliedacross the body is greater than the threshold torque value, wherebyafter disengagement of the torque limiting member at least a portion ofthe first member can rotate relative to at least a portion of the secondmember.

The invention also provides a work string assembly for use in an oil orgas well, the work string having a first tubular and a second tubular,and a torque limiting apparatus as defined above connected in thestring.

The invention also provides a method of treating wellbore in an oil orgas well, where the wellbore has an upper portion and a lower portion,the method comprising inserting a work string assembly into the wellborebeing treated, the work string assembly having upper and lower portionsof the work string, and at least one torque limiting apparatus; andwherein the method includes the step of injecting fluids through thework string assembly to displace fluids in the wellbore and allowingrelative movement of the upper and lower portions of the work stringassembly when the torque between the upper and lower portions of thework string assembly exceeds a threshold torque value.

According to one embodiment of the present invention there is provided awork string assembly for use in an oil or gas well, the work stringhaving a first tubular and a second tubular, with a torque limitingdevice connected in the work string, the torque limiting devicecomprising a substantially cylindrical body having an axial boretherethrough for fluid flow; the body comprising a first member and asecond member interconnected to be relatively rotatable; the firstmember including a first surface having at least one recess thereupon;the second member having at least one movable protrusion, eachprotrusion being biased by a resilient device and being movably arrangedin the second member such that the resilient device biases theprotrusion into the recess on the first member to cause the first andsecond members to rotate together; and wherein the recess and theprotrusion have slidably engageable surfaces such that when a torqueapplied across the body is greater than a preset load of the resilientdevice, the protrusion will disengage from the recess and at least aportion of the first member will rotate relative to at least a portionof the second member.

Typically the torque limiting device is connected in the work stringbetween the two tubulars, but in some embodiments the torque limitingdevice can be provided in one of the tubulars and can be spaced awayfrom the connection between the two tubulars, either above theconnection or (typically) below the connection between the two tubulars.

Typically the work string has a selectively actuable circulation portprovided in the string adjacent to and above the torque limiting device,whereby when the work string assembly is located in use in a borehole,opening of the port allows the annulus between the work string and thecasing to be flushed with fluid from the port.

Optionally the protrusion can comprise a piston. Optionally the devicecan have an axis, and the protrusion can move axially parallel to theaxis.

In some embodiments the device can be used in a string with a crossoveror step in the diameter, but it is equally useful in a string ofconsistent diameter along its length.

The device can be run on a rotating string with fluid flow therethrough.When torque is applied to the string, the device can disengage at apreset load and the first and second tubulars can freely rotate relativeto each other. Additionally, when the applied torque drops below thepreset load, the device can re-engage and the first and second tubularscan again rotate as a single unit. Thus the device can reset downhole,typically allowing repeated engagement and disengagement withoutintervention.

In some embodiments, the device can be arranged to disengage at a presetload, and then stay in that disengaged configuration, withoutre-engaging after the torque drops below the preset load.

Optionally, there are a plurality of protrusions and a matchingplurality of recesses. This can spread the load between the members andprovide a smoother engagement and disengagement of the tool.

Optionally the slidably engageable surfaces are rounded. Optionally thesurface of the recess substantially matches the surface of theprotrusion with which it engages. The surfaces may be part spherical. Inthis way a ball fitting can be used at an end of the protrusion.

Optionally the resilient device can be a spring, such as a mechanicalspring, or alternatively a gas spring. Optionally the protrusion and theresilient device are retained in a housing, and are typically arrangedto move axially with respect to the second tubular member. Arranging theprotrusion and the resilient device together in a housing helps them towork in unison. By arranging the housing and the resilient deviceaxially with respect to the second tubular member, the protrusions canbe constrained to move only axially with respect to the second memberand thus they can operate in a cam action to engage and disengage fromthe recess(es) when rotated relative thereto. Also, the resilient device(e.g. a spring) can be set in a long axial cavity and can be selected tohave a suitable spring rate to apply a high force with little axialmovement of the protrusion.

In some embodiments of the device the protrusion and resilient devicecan be arranged radially rather than axially.

Optionally a sleeve is arranged over the housing. In this way, thepiston and recess can be sealed off from the outside of the device.

Optionally, the resilient device comprises a spring, typically a discspring or a stack of such springs. Optionally there is a plurality ofdisc springs. By using disc springs the travel required to move betweenengaged and disengaged configurations is kept small. The number andrating of the springs can be is selected to determine the torque atwhich the device will disengage.

Optionally a stack of disc springs such as Bellville washers can bearranged in a cylindrical bore through the housing, which can optionallybe arranged to be parallel to and in alignment with the axis of thethroughbore of the device.

According to another embodiment, the invention also provides a torquelimiting device for use in a work string for an oil and gas well, thetorque limiting device comprising a body having an axial boretherethrough for fluid flow; the body comprising a first member and asecond member interconnected to be relatively rotatable; the first andsecond members being interconnected by a frangible member adapted totransfer torque between the first and second members up to a thresholdtorque value, thereby rotationally coupling the first and second memberstogether at torque levels below the threshold torque value, and whereinthe frangible member is adapted to break when torque applied across thebody is greater than the threshold torque value, whereby after breakingof the frangible member at least a portion of the first member canrotate relative to at least a portion of the second member.

Optionally the torque can be transferred through the frangible element,which can optionally comprise a pin or plate that is adapted to resistrelative rotation between two parts (e.g. two ends) of the torquelimiting device below a determinable torque threshold, but to shear orotherwise break above the determinable torque threshold, therebyallowing relative rotation between the two part of the torque limitingdevice. Optionally more than one frangible element can be provided, e.g.2, 3, 4 or more. The threshold torque value can be determined by thematerial of the element, the thickness of the element, the length of theelement, and the number of the elements.

Typically the frangible element is an elongate member that is arrangedparallel to the axis of rotation of the first and second members, forexample in the form of an elongate plate. Providing the frangibleelement in this form can optionally increase the surface area of thefrangible element that is engaged with each of the first and secondmembers, which can optionally be engaged along the elongate sides of theplate, providing a larger bearing area for transmission of forcesthrough the frangible element, and thereby reducing the risk ofpremature shearing of the frangible element. Also, the provision of thefrangible element in an elongate form reduces the tendency of thematerial of the frangible element to undergo surface and other damagesuch as brinelling.

Optionally the two parts of the torque limiting device can be providedwith bearings to reduce friction during relative rotation of the twoparts. Optionally the bearings can be lubricated by fluids passingthrough the device, which can optionally incorporate fluid pathways toguide a proportion of the fluid conducted through the torque limitingdevice through the fluid pathways to lubricate the bearings.

Optionally a resilient device can be provided to apply a compressiveforce between the two parts of the torque limiting device. Typically thebearings can be preloaded by the resilient device.

Optionally a bearing is located between the first and second tubularmembers. This typically provides smooth rotation of the members relativeto each other. In one embodiment the bearing is located between thesleeve and one of the tubular members, the sleeve typically beingconnected to the other member.

Typically the first and second tubulars can have different mechanicalproperties. In one embodiment of the invention, the first (upper)tubular can be stronger and more resistant to torque than the second(lower) tubular.

Typically the work string can be a tapered string. For example, thefirst and second tubulars can be of different sizes, e.g. of differentdiameters. In one embodiment of the invention, the first (upper) tubularcan have a larger diameter than the second (lower) tubular. Thedifference in diameter can typically result in a difference inmechanical properties. In some embodiments the two tubulars can be ofsubstantially the same outer diameter, and the work string can beprovided with a substantially consistent outer diameter across thetorque dislocation device.

In certain embodiments of the invention, the upper tubular can have asmaller diameter than the lower tubular.

Optionally the casing being treated with the work string can have a stepin its inner diameter, although the work string is optionally useful fortreating casing that has a substantially consistent inner diameter.

According to a further embodiment, the present invention also provides amethod of treating a liner portion located in a well bore, comprisingthe steps:

-   -   (a) locating a torque limiting device in a work string;    -   (b) running the work string in the well while rotating the        string and the torque limiting device;    -   (c) circulating a fluid through the string and up an annulus        between the string and the liner to the liner portion to be        treated    -   (d) allowing the torque limiting device to disengage under the        applied torque in the liner; and    -   (e) passing fluid through the casing at the liner portion.

The liner portion can optionally be the top of the liner.

In some embodiments, the work string can optionally incorporate acirculation tool, although this is not necessary in all embodiments. Thecirculation tool if present is typically located in the region of theliner portion to be treated, optionally above the torque limitingdevice, and typically being opened to pass fluid from the bore of thework string though the circulation tool and into the annulus between thestring and the liner.

In this way, the treatment fluid, e.g. the cleaning fluid can reach e.g.the liner top before the fluid is pumped through the circulation tool,providing for efficient cleaning of the liner top.

The method typically includes cleaning or washing of the wellbore.

The wellbore can optionally be cased.

Optionally the method further comprises the step of re-engaging thetorque limiting device. In this way the circulation tool can typicallybe moved easily to operate at another location in the well whilerotating the tool string.

According to a further embodiment of the invention there is provided amethod of treating a wellbore in an oil or gas well, where the wellborehas an upper portion and a lower portion with a connection between them,the method comprising inserting a work string into the wellbore beingtreated, the work string having upper and lower portions, and a torquedislocation device connected in the work string; and wherein the methodincludes the step of injecting fluids through the work string todisplace fluids in the wellbore and allowing relative movement of theupper and lower portions of the work string when the torque between theupper and lower portions of the work string exceeds a predeterminedthreshold.

Typically the torque dislocation device allows relative rotation of theupper and lower portions of the work string when the torque between theupper and lower portions of the work string exceeds a predeterminedthreshold.

Typically the upper portion of the wellbore has a larger inner diameterthan the lower portion.

Typically the method includes landing the torque dislocation device inthe region of the connection between the upper and lower portions of thewellbore being treated.

Typically the work string can have cleaning tools such as brushes and/orscrapers provided in the work string, typically below the torquedislocation device, and typically spaced above the bottom of the string.Typically other wellbore cleaning tools e.g. brushes and scrapers etccan also be provided above the torque limiting device adjacent to thecasing above the liner.

Typically the work string can have a circulation port located in thework string, optionally above the torque dislocation device.

Typically the work string can have a filter device.

Typically the work string can have a junk catcher device.

Typically the work string can comprise a fishing tool and can be used ina fishing operation to recover dropped or lost items or “fish” from thewellbore. Optionally the fishing tool can include a milling tool to cutthe fish typically by a rotary action applied through rotation of thework string, in order to facilitate recovery of the fish. The millingtool can be a packer mill or the like. In certain embodiments, thefishing tool can include a wash pipe or overshot or some other fishingtool that engages the outer diameter of the fish. Alternatively, oradditionally, the fishing tool can comprise a spear or tapping tool thatengages the inner surface of the fish. The work string can incorporatejarring tools in some embodiments. Any suitable fishing tool or jarringtool can be incorporated into the work string, below the torque limitingdevice. Suitable fishing and jarring tools are well known in the art andsome examples are shown in the Weatherford publication “FishingTechnology Best practices Manual” the disclosure of which isincorporated herein by reference.

The torque limit setting of the torque limiting device is typically setto disengage and allow relative rotation of the components on oppositesides of the torque limiting device before the torque in the work stringapproaches the limits of the portion of the string below the torquelimiting device, typically the wash pipe or other fishing tool, whichtypically has less resistance to torque than the drill pipe or othertubular above the torque limiting device. Therefore the torque limitingdevice typically maintains the operating torque of the work string as awhole within the limits of the weakest component in the string, therebyreducing the risks of torque related damage to the weaker componentseven if torque above the limiting device occasionally rises above theoperational limits of the lower string.

Optionally more than one torque limiting device can be incorporated intothe string, for example 2, 3, 4 or more torque limiting devices can beincorporated into the string. Optionally the torque limiting devices canbe spaced apart from one another in the string, or can be groupedtogether, or clusters of torque limiting devices can be grouped together(e.g. 2, 3, 4 or more adjacent devices per cluster) while the clustersare spaced apart along the string. Optionally the torque limitingdevices can be set to disengage (and thereby allow relative rotationbetween the string above and below each respective torque limitingdevices) at different thresholds of torque. Thus the torque limitingdevices set at different parts of the string can be set to disengage atdifferent torque thresholds, so that as the torque experienced by thestring increases, different section of the string disengage in a safemanner. Simple embodiments can be constructed with e.g. two torquelimiting devices, which are typically set to dislocate at thresholdtorque values that are different from one another, e.g. one lowthreshold and one high threshold. Optionally the low threshold devicecan be located below the high threshold device, and can optionally belocated relatively near to the lower end of the string, so that theoperator can be relatively confident that the lower torque limitingdevice can be expected to dislocate at a relatively low torquethreshold. Further, the operator can also be reasonably confident thatthe event of the lower device dislocating will not significantly alterthe total torque in the string and so will not significantly affect theconfiguration of the other, higher threshold torque limiting device atthe top of the string.

In some other embodiments incorporating more than one torque limitingdevice, optionally the torque limiting devices set at different parts ofthe string can be set to disengage at the same torque threshold, so thatonce that threshold is reached, spaced apart sections of the stringdisengage at the same time.

Thus, when using more than one torque limiting device, the thresholdtorque limits for each device may be set to different values, but inother embodiments using more than one limiting device at differentlocations within a workstring, the threshold torque limits for eachdevice may be set to the same or similar values.

In embodiments with more than one torque limiting device, the devicescan be the same or different. For example, in some such embodiments, onetorque limiting device set to rotationally disengage two parts of thestring at a particular torque threshold can optionally re-engage the twoparts and permit the transfer of torque across the connection when thetorque level drops back down below the threshold torque value. This canoptionally happen automatically. The string can include other torquelimiting devices that are also re-engagable in this manner, optionallyautomatically. Such strings can also include “one-shot” torque limitingdevices that are not re-engagable, and/or that are not automaticallyre-engagable, and in which, after the torque threshold has been reachedand the device has disengaged the two parts of the string, the devicecannot automatically reset in situ, so that when the torque levelthrough the connection drops back below the threshold, the deviceremains disengaged, preventing the transfer of torque across theconnection, requiring intervention to re-set the one-shot device(s).Such composite strings including re-engagable and “one-shot” devices canhave 1, 2, 3, 4 or more of each kind of torque limiting device,optionally spaced apart from one another in the string, and each can beset to the same or different torque thresholds. The one-shot device(s)can be above or below the re-engagable devices in the string.

The invention also provides an assembly for use in a wellbore,comprising: a workstring;

an upper torque limiting device comprising:a first member connected to a second member such that torque applied tothe first member below a first threshold torque value is transferred tothe second member, wherein torque applied to the first member above thefirst threshold torque value is not transferred to the second member;a lower torque limiting device comprising:a third member connected to a fourth member such that torque applied tothe third member below a second threshold torque value is transferred tothe fourth member, wherein torque applied to the third member above thesecond threshold torque value is not transferred to the fourth member.

The first threshold torque value can be substantially equal to thesecond threshold torque value, or can be different, e.g. typicallygreater than the second threshold torque value.

The various aspects of the present invention can be practiced alone orin combination with one or more of the other aspects, as will beappreciated by those skilled in the relevant arts. The various aspectsof the invention can optionally be provided in combination with one ormore of the optional features of the other aspects of the invention.Also, optional features described in relation to one embodiment cantypically be combined alone or together with other features in differentembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a cross sectional view through a torque limiting deviceaccording to an embodiment of the present invention;

FIG. 2 is an enlarged view of a portion of the device of FIG. 1;

FIG. 3 is part sectional view of the portion of the device of FIG. 2;

FIG. 4( a) is a perspective view of a lower end of a first tubularmember of FIG. 1 and FIG. 4( b) is a perspective view of the upper endof a housing in which the pistons locate;

FIGS. 5( a) and 5(b) are perspective views of a piston for locating inthe housing of FIG. 4( b) while slidably engaging with the recesses inthe first tubular member of FIG. 4( a);

FIG. 6( a) is a perspective view of a top end of the second tubularmember and FIG. 6( b) is a perspective view of the lower end of thehousing that receives the top end;

FIG. 7 provides a schematic illustration of a torque limiting devicebeing used in a method of cleaning a liner top according to anembodiment of the present invention;

FIG. 8 shows a further example of a torque limiting device used in afishing string;

FIGS. 9 and 10 show a schematic view of further examples of a torquelimiting device used in a cleaning and/or scraping string; and

FIGS. 11, 12, and 13 show side and close up views of a further exampleof a torque limiting device that can be used in a workstring.

DETAILED DESCRIPTION

Various embodiments and aspects of the invention will now be describedin detail with reference to the accompanying figures. Still otheraspects, features, and advantages of the present invention are readilyapparent from the entire description thereof, including the figures,which illustrates a number of exemplary embodiments and aspects andimplementations. The invention is also capable of other and differentembodiments and aspects, and its several details can be modified invarious respects, all without departing from the spirit and scope of thepresent invention. Accordingly, the drawings and descriptions are to beregarded as illustrative in nature, and not as restrictive. Furthermore,the terminology and phraseology used herein is solely used fordescriptive purposes and should not be construed as limiting in scope.Language such as “including,” “comprising,” “having,” “containing,” or“involving,” and variations thereof, is intended to be broad andencompass the subject matter listed thereafter, equivalents, andadditional subject matter not recited, and is not intended to excludeother additives, components, integers or steps. Likewise, the term“comprising” is considered synonymous with the terms “including” or“containing” for applicable legal purposes.

Any discussion of documents, acts, materials, devices, articles and thelike is included in the specification solely for the purpose ofproviding a context for the present invention. It is not suggested orrepresented that any or all of these matters formed part of the priorart base or were common general knowledge in the field relevant to thepresent invention.

In this disclosure, whenever a composition, an element or a group ofelements is preceded with the transitional phrase “comprising”, it isunderstood that we also contemplate the same composition, element orgroup of elements with transitional phrases “consisting essentially of”,“consisting”, “selected from the group of consisting of”, “including”,or “is” preceding the recitation of the composition, element or group ofelements and vice versa.

All numerical values in this disclosure are understood as being modifiedby “about”. All singular forms of elements, or any other componentsdescribed herein are understood to include plural forms thereof and viceversa.

FIG. 1 of the drawings illustrates a torque limiting device generallyindicated by reference numeral 10, according to an embodiment of thepresent invention. At an upper end 12 there is located a top sub 14having a box section 16 for connecting a tubular of a tool string to thedevice 10. A sleeve 20 is fixed around a lower end 18 of the top sub 14by a screw thread and an o-ring seal is provided between the innersurface 22 of the sleeve 20 and the outer surface 24 of the top sub 14.The sleeve has an inwardly directed upward facing circumferential ledge32 on the inner surface 22 of the sleeve 20.

Held within the sleeve 20 is a bottom sub 26. The upper end 28 of thebottom sub 26 has an outwardly directed downward facing circumferentialledge 30 that is arranged axially opposite to the ledge 32, so that theupper end 28 of the bottom sub 26 is retained between the lower end 18of the top sub 14 and the upper surface of the ledge 32. A bearing ring34 is located within the circumferential chamber formed between theledges 30,32, the inner surface 22 of the sleeve 20 and the outersurface 36 of the bottom sub 26.

Bottom sub 26 typically terminates in a pin section 38 at a lower end40. The pin section 38 is connected to a box section 42 of a cross-oversub 44 as is known in the art. The cross-over sub 44 is used to providean upwardly directed ledge 46 facing a bottom end 48 of the sleeve 20.The ledge 46 and bottom end 48 retain a second bearing ring 50rotationally mounted on the outer surface 36 of the bottom sub 26. Arotary seal 52 is also located between the first and second bearingrings 34,50 between the inner surface 22 of the sleeve 20 and the outersurface 36 of the bottom sub 26.

The bottom sub 26 is therefore constrained within the sleeve 22, whichremains rotationally fixed to the top sub 14 by means of the screwthread. The top sub 14 and bottom sub 26 are rotationally connectedtogether by a torque limiting connection 15 operating between them. Thetorque limiting connection 15 is provided between the opposing surfacesof the upper sub 14, which is rotationally fixed to the upper part ofthe string, and the lower sub 26, which is rotationally fixed to thelower part of the string and is rotatable relative to the top sub athigh torque loads that dislocate the connection 15.

The torque limiting device 10 of FIG. 1 can typically be incorporatedinto a cleaning string, having a circulating device above the torquelimiting device 10 and a cleaning tool below it. Typically the torquelimiting device would be positioned at the lower end of an upper stringof drill pipe having a relatively high resistance to torque, typicallyhigher than the cleaning string below the device 10. Positioning thetorque limiting device 10 at the top of the lower string has theadvantage that the device 10 protects all the connections in the lowerstring, so that if the torque values applied through the connectionbetween the upper string and the lower string were to momentarily exceedthe limits of the lower string then the torque limiting device woulddisengage to avoid transferring the high torque values above it to theweaker cleaning string below it.

Wellbore cleaning operations are typically carried out in sections ofcasing that have been cemented in place to provide a secure conduit andto seal off the formation. The cleaning string incorporating the torquelimiting device is typically run into the wellbore to clean the insideof the casing, although this embodiment is equally useful for cleaningother well profiles and configurations and the inner surface of the holecan take many different forms including casing, liner and also an openhole section. In a typical embodiment, the cleaning string is deployedto clean a casing and liner configuration having a step in the internaldiameter at the top of the liner, which typically requires two differentsizes of work string/drill pipe in order to effectively clean thecomplete wellbore.

In this example, when the well is cased and lined, and the casing andliner are full of drilling mud, the cleaning string is made up at thesurface incorporating the torque limiting device and the liner cleaningtools below it and is run into the well, with the appropriate length ofsmall diameter tubular comprising the lower string beneath the torquelimiting device, which has been measured before being made up and runinto the well. The torque limiting tool 10 is typically installed at thevery uppermost end of the lower “skinny” string so that the top end ofthe torque limiting tool 10 typically connects directly onto the upperstring of larger diameter drill pipe or similar. There are typically no“skinny” pipe components above the torque limiting tool in the presentembodiment. The upper string above the torque limiting device 10 cantypically have a circulation tool or similar and may incorporate furthercasing cleaning tools adapted to clean the casing above the liner. Theseare typically made up to the top connection of the torque limiting tooland run into the well. The upper string is then made up using typicallydrill pipe and the whole assembly is run into the hole to the requireddepth. At this point the pumps at the surface are typically activated tocirculate the well with mud to remove any debris released during therunning in phase. The circulation through the work string continuesuntil the mud returns from the well are clean. At that stage, theoperator can optionally pump cleaning chemicals such as detergents,scourers etc through the string before pumping completion fluid such asbrine. Fluid returns from the well are typically monitored continuallyduring these pumping phases, and quality checks are carried out oncompletion fluid returning to surface. When the returning completionfluid is of an acceptable quality the pumps are stopped and the assemblyis pulled out of the hole. Typically the cleaning tools are visuallyinspected when they return to surface to look for signs of wear ordamage, and the conventional completion operations can then be carriedout.

Pipe movement, typically rotation, is useful in effective removal ofdebris particularly from the lower side of deviated casing and liner.The torque limiting device facilitates maximum benefit in terms ofdebris removal from the well through maximum rotational pipe movement inthe knowledge that the torque limiting tool will activate downholeshould the downhole torque become excessive as completion fluid isdisplaced into the annulus. This mitigates the risk of downhole failureof a drill pipe connection which may result in a costly fishingoperation to retrieve the part of the wellbore cleaning assembly anddrill pipe left in the well.

During the cleaning operation the string is rotated from the surfacedrive. The torque limiting device 10 typically operates passively tode-couple the upper and lower strings from one another when the torquepassing through the tool exceeds the preset value thus disengaging thelower string. The torque limiting device typically re-engages once thetorque levels in the string drop below the limit. Typically the limit isset to de-couple the string at a threshold below the upper limit of thelower “skinny” string so that no torque is transmitted to the lowerstring above the threshold that it can bear.

During cleaning operations it is only possible to monitor the totaltorque reading for the complete string from the surface, and is notpossible to measure or monitor cumulative torque in real time at anyparticular point in the string. It is possible however to predict thedownhole torque at any point on the string and therefore estimate whereand when an excessive torque situation is likely to occur. Whether thetool activates or not on an actual job can be checked by monitoring thetotal torque reading at the surface. In the event the torque limitingtool activates downhole, the total observable torque at the surface willreduce and this will be manifested by a sudden drop in the surfacetorque value confirming likely de-coupling of the tool and relativerotation of the upper and lower strings.

Reference is now made to FIG. 2 of the drawings which shows an enlargedview of the arrangement of elements of the torque limiting connection 15between the lower end 18 of the top sub 14 and the upper portion 28 ofthe bottom sub 26. Located at the upper end 28 of the bottom sub 26 isan axial stack of disc springs 56 typically in the form of Bellvillewashers, with a central bore and an annular ring. The disc springs 56are typically held in compression in an axial stack between the upperend 28 of the bottom sub 26 and the inner surface of a cylindricalhousing 58 that fits over the upper end 28 of the bottom sub 26. Thusthe housing 58 is retained on the end 28 of the sub 26 and the springstack 56 urges it upwards.

The cylindrical housing 58 typically has an upwardly facing generallyflat circular end face 62 at an upper end 60 as best seen in FIG. 4( b).The flat end face 62 has a central axial bore 64 located therethrough,which continues axially through the entire length of the device 10 toprovide a fluid passageway through the device 10. The housing 58 hasthree cylindrical bores 66 a-c extending axially parallel to the axis ofthe housing and the bore 64, and emerging from the flat end face 62. Thebores 66 are typically blind bores emerging from the face 62 atlocations that are typically equidistantly spaced circumferentiallyaround the bore 64, and can typically be radially spaced approximatelymid way between the bore 64 and the edge of the flat end face 62.

In the lower end 18 of the top sub 14, as best illustrated in FIG. 4(a), there is provided a downwardly facing generally flat circular endface 68. The face 68 is recessed at 70 a-c at radial and circumferentialpositions matching the recesses 66 a-c on the face 62 of the housing 58.The central bore 64 extends through the top sub 14 and is aligned withthe central bore 64 extending through the bottom sub 26, allowing fluidcommunication through the device 10.

Bores 66 a-c extend axially through the housing 58 and are typicallysized to retain protrusions in the form of pistons 72 on the upwardlyfacing end face 62. An embodiment of a piston 72 is illustrated in FIGS.5( a) and 5(b), and typically has a lower end 74, having a planar face76, which fits into one of the bores 66 as illustrated in FIG. 3. Theupper end 78 of piston 72 typically has a generally hemi-spherical upperface 82 which typically has a profile of a flattened cone whichadvantageously extends proud of the housing surface 62 when the lowerend 74 is engaged with the base of the bore 66.

The dimensions of the piston 72 and particularly the hemi-sphericalprofile of the upper face 82 thereof typically complement the innersurfaces of the recess 70 of the top sub 14 so that the upper face 82engages closely with the inner surface 80 of the top sub 14. Typicallythe upper face 82 and inner surface 80 are formed with chamfered edges.The arrangement is such that the piston 72 can be received in the recess70 to rotationally lock the two components together at low torque loads,and to allow the transmission of relatively low torque loads betweenthem. Thus the housing 58 and top sub 14 can be rotationally coupled atlow loads. However, when relatively high torque loads are transmittedacross the connection 15, the piston 72 can slide circumferentially outof the recess 70 to disconnect the two components from one another. Theforce connecting the two components together can be influenced by theselection of the mating profiles of the recesses 70 and the pistons 72,and by the force of the spring stack 56 that urges the housing 58upwards against the top sub 14.

When the piston 72 is located in the recess 70 the device 10 is in anengaged configuration and the housing 58, bottom sub 26 and top sub 14rotate together at low torque. As torque is increased from the top sub14 to the bottom sub 26, it is transferred through the connection 15until a threshold torque value is reached at which the torsional forcebeing transmitted through the connection 15 is higher than the forceapplied to the housing 58 from the spring stack 56. At this point, theforce applied by the spring stack 56 is insufficient to maintain theconnection 15, and the piston 72 slides circumferentially out of therecess 70 thereby disengaging the connection 15 before the high torqueloads transmitted through it damage the string below the connection 15.When the pistons 72 are not engaged in the recesses and no torque isbeing transmitted across the connection 15 the device is in a disengagedconfiguration. Continued relative rotation at high torque moves thepistons around the lower surface 68 of the top sub 14, withoutre-engaging the transmission. However, if the torque applied from thetop sub 14 to the bottom sub 26 decreases below the threshold torquevalue determined by the shape of the pistons and the force of the springstack 56, then the device 10 can itself move back to the engagedconfiguration by locating the piston 72 in a recess 70 on it's patharound the surface 68, thereby allowing the transmission of lower torquewithout having to reset the device 10.

Each piston 72 is biased axially so that it will tend to stay engaged ina recess 70 when aligned with one. The bias comes from the disc springs56 which act on an internal surface 84 of the housing 58 directly behindthe bores 66. To prevent any build-up of pressure between the pistons 72and the recesses 66, each recess 66 has a port 86 drilled through itsbase 88 to the surface 84. The disc springs 56 are therefore selected toprovide a load upon the pistons 72 such that each piston 72 will remainseated in the recess 70 until a torque is applied which is greater thanthe load applied by the springs 56, at which point the pistons 72 willslide from the recesses 70 and the connection 15 will disengage.

In order that the pistons 72 and housing 58 do not rotate with the topsub 14 when disengaged, the housing 58 is typically rotationally fixedto the bottom sub 26, typically by rods 90 best shown in FIGS. 6 and 3.The upper end 28 of the bottom sub 26 has a reduced diameter neck 92over which the housing 58 can slide. Typically the circumferential outersurface of the housing 58 has the same diameter as the outer surface 36of the bottom sub 26. Arranged circumferentially around the outersurface of the neck 92 are axially extending elongate grooves 94 a-f ofsemi-cylindrical cross-section. Axially extending elongate grooves 96a-f typically of matching semi-cylindrical cross-section are milled onan inner surface 98 of the housing 58 at the lower end 100. The grooves94 and 96 align when the housing 58 is assembled and cooperate to retainthe rods 90 between them, typically while allowing the bottom sub 26 andhousing 58 to move axially relative to each other under the force of thesprings 56.

The device 10 is typically assembled by locating the disc springs 56within the housing 58 and inserting the pistons 72 into the bores 66 onthe top of the housing 58, which is then slid onto the neck 92 at theupper end 28 of the bottom sub 26, with the rods 90 retained between thegrooves 94, 96. The lower face 68 on the base of the top sub 14 is thenpressed against the upper face 62 of the housing 58 so that the pistons72 align with the recesses 70 and engage within them The bearing ring 34is located at the ledge 32 of the bottom sub 26. The sleeve 20 is thenslid over the bottom sub 26 and affixed to the top sub 14 by engagingthe screw threads at the lower end 18 of the top sub 14 and the upperend 102 of the sleeve 20. The bearing ring 34 is positioned to allow asmall amount of axial movement between the ledges 30,32 to take up thecompression and expansion in the disc springs 56. The bearing ring 40 islocated against the bottom end 48 of sleeve 20 over the bottom sub 14and then crossover 44 is screwed onto the pin section 30 of the bottomsub 26. When assembled the device 10 provides a substantiallycylindrical body with standard box section 16 and pin section 104fittings for connection to other tools or tubulars in a string.

In use, the device 10 is mounted in a string using the box 16 and pin104 sections. The pistons 72 are biased into the recesses 70 by thesprings 56 and the device 10 is in the engaged configuration. Rotationof the top sub by low torque loads 14 will rotate the sleeve 20rotationally fixed thereto and torque is transmitted through the rods 90to drive rotation of the bottom sub 26 and crossover 44. The device 10will thus rotate uniformly along its entire length at low torque values.

At higher torque values, such as might occur if the string below thecrossover 44 experiences high drag through rotation in a dense fluid,the torque will eventually increase to a threshold torque value thatdisengages the connection device 15. Below the threshold torque value,the top and bottom subs 14, 26 will rotate together and transmit torquethrough the tool as described above. However, once the torquetransmitted through the connection device 15 rises above the thresholdtorque value, the force from the springs 56 keeping the pistons 72engaged with the recesses 70 is overcome by the torque transmittedthrough the connection 15, and the pistons 72 disengage from therecesses 70, thereby disengaging the connection 15. The pistons 72 areforced downwards against the bias of the springs 56, disengaging fromthe recesses 70 and slide circumferentially around the surface 68. Theaxial distance between the ledges 30,32 will reduce accordingly. In thisdisengaged configuration, the bottom sub 26 and crossover 44 rotaterelative to the top sub 14 and sleeve 20 on bearing 34, and no torque istransmitted across the connection device 15.

When torque applied to the upper sub 14 is reduced, or when dragexperienced by the lower string is reduced, the force applied to thepistons 72 by the springs 56 is eventually able to re-engage the pistons72 with the recesses 70 and reconnect the top and bottom subs 14, 26, totransmit torque across the connection 15 once more. Also, when the speedof rotation of the two subs 14, 26 approach one another, for example, byslowing of the upper sub 14, the pistons 72 will move across the surface68 and relocate in the recesses 70 by virtue of the spring bias. Thedevice 10 is then engaged again. It will be apparent that the device 10can be switched multiple times between configurations within the wellbore.

Reference is now made to FIG. 7 which illustrates a tool string,generally indicated by reference numeral 110, within a well bore 112 forthe purpose of cleaning a liner top 114, according to an embodiment ofthe present invention. Like parts to those in the earlier Figures willbe given the same reference numeral to aid interpretation.

As described above the torque limiting device 10 is assembled andconnected in a tool string 110. There will be a portion of tool string128 above the device 10 and a portion of tool string 130 below thedevice 10. Directly above the device 10 there is typically located acirculation tool 116.

Circulation tools 116 are well known in the art and principally comprisea set of ports 118 which can be opened to provide fluid communicationfrom the central bore 64 to an annulus 120 between the string 110 and acasing 122 or liner within the well bore 112. While a circulation tool116 is used in this embodiment it will be apparent that other clean-upfeatures can be provided in the tool 116. For example a filter tool 116can be used which would trap debris during the cleaning method.

The tool string 110 is typically run into the well bore 112 while thestring is rotated and mud is circulated. The mud is pumped down thecentral bore to an end (not shown) of the string 110, whereupon it exitsthe string 110 and is forced up the annulus 124, 120 to the surface (notshown) of the well bore 112.

The tool string 110 is run to a depth where the circulation tool 116 ispositioned at the liner top 114. The liner top 114 is found at the topof a liner 126 which has been hung from an upper casing 122 or liner.The tool string 110 may have been used for other operations within thewell bore 112 and then be repositioned at the desired position whencleaning is required. Alternatively a dedicated intervention run may bemade for the purpose of clean-up. During movement of the string 110,rotation is maintained and the device 10 is in the engaged configurationso that the rotation is transmitted down the entire string 110 throughthe tool 10.

The clean-up process is begun by pumping a cleaning fluid, typicallybrine, down the central bore 64 from the surface. The brine displacesthe mud within the bore 64 and the interface between the two will movedown the central bore 64, exit the string, and move up the annulus 124between the string 130 and the liner 126. Rotation of the string 110 ismaintained through this process, and as the torque is typically belowthe disconnection threshold, the connection 15 remains engaged and thestring 110 rotates as a whole.

When the interface reaches the liner top 114, a change in pressure isseen at the surface brine pumps as the brine enters the now widerannulus 120 between the string 128 and the casing 122. The weight of mudmay be sufficiently heavy to counter the pumping pressure from thebrine. In order to assist the process, the ports 118 on the circulationtool 116 are typically opened. Brine can then pass from the central bore64 to the annulus 120 at the liner top 114, mix with the existing brinebehind the interface and the additional pressure provided, as the pumpdoes not have to move the brine in the bore 64 below the liner top 114or in the annulus 124, thus the interface moves up the annulus 120 tothe surface. In this way, the entire wellbore 112 is flushed with brineand any debris, particles or mud will have been swept from the annulus120,124 to leave a clean well. In particular as the brine from the portsmixes with brine behind the interface at the liner top 114, the linertop 114 is also flushed with brine to ensure it is cleaned.

This is in contrast to prior art techniques which stop rotation of thestring 110 before the interface has reached the liner top 114. Pumpingof brine from the ports 118 will then leave a column of mud between theport 118 position and the interface, at the liner top 114, which willprovide inefficient cleaning. This procedure is used because maximumtorque will be applied to the string 110 at the point where theinterface reaches the wider annulus 120 and the ports 118 are opened.Such torque would damage the string 110 as it is rotated.

By using the device 10 of the present invention, the disc springs 56 canbe set to disengage the device 10 when the torque applied reaches a setthreshold, typically reached as the brine mud interface approaches theliner top 114. The string 128 above the device 10, including thecirculation tool 116 and the top sub 14 and sleeve 20 of the device 10,can then be rotated at a constant rate and torque from the top drive onthe rig, as fluid is pumped into the annulus 120 at the liner top 114.The torque applied to the string 110 by the top drive can be fairly highwithout risks of torsional damage to the string 110 or its components,because as the torque threshold is reached as a result of the upwardmovement of the mud-brine interface in the lower annulus 124, and thedrag of the lower string 130 increases to the point where the torqueapplied across the device 10 rises above the disconnection threshold,then the connection 15 will disengage allowing the bottom sub 16,crossover 44 and remainder of the string 130 below the device 10 toremain stationary and so prevent damage to the string 110 or thecrossover. Typically if the connection device 15 disengages, thedisconnection will be evident to the operator by a pressure spike in thesurface brine pumps and in the event that continued rotation of thelower string 130 is desired, for example, when the connection 15disengages before the mud-brine interface has reached the liner top 114,then the torque applied by the top drive can be reduced slightly to slowthe rotation of the upper string and to allow the connection device 15to re-engage as previously described so that the entire string 110rotates as a whole. Optionally the disengagement of the connection 15can be signalled to the surface by other ways, for example by signallines.

The disc springs 56 are typically chosen to compress at a calculatedexpected maximum torque at the point where the mud-brine interfacereaches the casing 120, but the device 10 also provides a safety featurein that, if the string 130 sticks at any point below the device 10,torque above the threshold will be transmitted to the device 10 andconsequently the device 10 will disengage, so preventing any damage tothe string 110, by stopping rotation of the string 130 relative to thestring 128. If the obstruction is cleared, say by increased pumping, thedevice 10 can be reengaged to provide through tubing rotation in theentire tool string 110 again.

A principal advantage of the present invention is that it provides atorque limiting tool which can be re-set between an engaged anddisengaged configuration any number of times within a well bore.

A further advantage of the present invention is that it provides amethod of cleaning a liner top which is more efficient than the priorart techniques, by allowing cleaning fluid to reach a liner top prior tojetting cleaning fluid at the liner top.

Referring now to FIG. 8, a further embodiment is shown in the form of afishing string 200 incorporating a torque limiting device generallyindicated by reference numeral 210. Above the torque limiting device 210there is located a top string 214 typically composed of drill pipeconnecting to the upper end of the torque limiting device 210. Thetorque limiting device can be essentially as described in the previousembodiments, and the fishing string 200 can optionally incorporate acirculation sub 214 c above the torque limiting device 210, within thestring of drill pipe.

Below the torque limiting device 210 the fishing string 200 can have avariety of known fishing tools such as mills, e.g. packer mills, washpipes, spears, screw in subs, overshots and tapping tools, and otherfishing tools including any combination of more than one of these.Typically the tools are operated by rotation of the string 200 from thetop drive on the rig.

Between the fishing tools and the torque limiting sub the fishing string200 can optionally have one or more known jarring tools, bumper subs,intensifiers and/or stabilisers, or combinations of the same. Optionallythe jarring and bumper tools could be provided above the torque limitingdevice 210. Elsewhere in the string 200 recovery subs such as bootbaskets etc can be provided to retain and recover large pieces of debrisgenerated during the fishing and milling operations.

Thus the torque limiting device can be incorporated into a fishingstring, typically having a circulating device above the torque limitingdevice 210 and a string of fishing tools 244 below it. Typically thetorque limiting device 210 could be positioned at the lower end of theupper string of drill pipe having a relatively high resistance totorque, typically a higher torque resistance than the fishing stringbelow the torque limiting device 210. Positioning the torque limitingdevice 210 at the top of the lower string has the advantage that thedevice 210 protects all the connections in the lower string, so that ifthe torque values applied through the connection between the upperstring 214 and the lower string 244 were to momentarily exceed thelimits of the lower string 244 then the torque limiting device woulddisengage to avoid transferring the high torque values above it to theweaker fishing string below it. For example some fishing tools are veryfragile having very low capacity to resist torque. Wash pipe for exampletypically has a very thin wall to fit between the inner surface of thecasing and the outer surface of the fish. It therefore has a lowercapacity to resist torque than the drill pipe in the upper string 214.

Fishing operations are typically carried out in sections of casing oropen hole. The fishing string 200 incorporating the torque limitingdevice 210 is typically run into the wellbore to try to recover a stuckfish 250. In some cases, the fish 250 is stuck below a casing or linerjoint having a step in the internal diameter at the top of the liner.

In this example, the fishing string 200 is made up at the surfaceincorporating the torque limiting device and the fishing tools below itand is run into the well. The torque limiting tool 210 is typicallyinstalled at the very uppermost end of the lower and weaker string 244so that the top end of the torque limiting tool 210 typically connectsdirectly onto the upper string of larger diameter drill pipe 214 orsimilar. There should typically be no “skinny” pipe components above thetorque limiting tool. The upper string 214 above the torque limitingdevice 210 can typically have a circulation tool 214 c or similar andmay incorporate casing cleaning tools adapted to clean the casing abovethe liner. These are typically made up to the top connection of thetorque limiting tool and run into the well as the string 200 is beingmade up. The upper string 214 is then made up using typically 5″ drillpipe and the whole assembly is run into the hole to the required depth.

The fishing tools in the lower string 244 below the torque limitingdevice 210 are then operated from surface in conventional ways to catchand recover the fish.

During the fishing operation the string is rotated from the surfacedrive to operate the tools in the lower string 244. The torque limitingdevice 210 typically operates passively to de-couple the upper 214 andlower 244 strings from one another when the torque passing through thetool 210 exceeds the preset value thus disengaging the lower string 244from the upper 214. The torque limiting device 210 typically re-engagesonce the torque levels in the string 200 drop below the limit. Typicallythe limit is set to de-couple the string at a threshold below the upperlimit of the lower “skinny” string 244 so that no torque is transmittedto the lower string 244 above the threshold that it can bear.

During fishing operations the total torque reading for the completestring can be monitored from the surface and plotted to identify suddendrops in torque which would indicate likely de-coupling of the tool andrelative rotation of the upper 214 and lower 244 strings.

A further two examples of a torque limiting tool incorporated in acleaning string are shown in FIGS. 9 and 10. In earlier embodiments, thetorque limiting tool is typically placed at or near to a junction in thestring between larger diameter and smaller diameter parts of the string,and in particular, desired locations for the torque limiting device inearlier embodiments have been at or near the top of the smaller diameterstring. Positioning the torque limiting device at this location isdesirable in the earlier embodiments in order to protect the lower,smaller diameter string below the torque limiting device. A typicalexample might involve a step in the inner diameter of the casing from,e.g. 9.625″ casing down to 7″ liner. Typically wellbore cleaningoperations performed at such sites use a work string having cleaningand/or scraping tools mounted on 5″ drill pipe (DP) with NC50connections in the 9.625″ casing and 3.5″ DP with NC38 connections inthe 7″ liner. Hence there is a step in the diameter of the cleaningstring, and the lower string of 3.5″ DP has less torsional capacitybefore a failure threshold than the larger diameter section of 5″ DP. Insuch cases, according to earlier examples of the invention, it wasdesirable to located the torque limiting tool at the very top of the3.5″ DP, typically at the transition between the larger 5″ and thesmaller 3.5″ diameters of DP, in order to protect the lower capacity3.5″ string by dislocating the two sizes of DP from one another at thejunction in the event that the torque in the cleaning string as a wholeexceeded the torsional capacity of the lower 3.5″ string. In theembodiments of the invention described in FIGS. 9 and 10, the torquelimiting device is not necessarily placed at or near to the transitionbetween the different diameters. It has been found by the inventors thatpositioning the torque limiting tool at other locations, e.g.mid-string, so that part of the 3.5″ section of DP is below the torquelimiting device, and part is above it, is also effective in combatingthe tendency of the string to fail if the torque exceeds the torsionalcapacity of the lower section of 3.5″ DP.

FIG. 9 shows schematically the downhole end of a 5000 foot liner 320 inwhich a work string 300 is operational with cleaning, brushing, and/orscraping tools 301 located above a drill bit or mill 302. A torquelimiting device 310 is located mid-way down the lower section of 3.5″ DP300I, spaced away from the interconnection with the upper section of 5″DP 300 u. Torque calculations based on design data models indicate thatwhen the low viscosity drilling mud surrounding the lower section 300Iof 3.5″ DP is displaced with brine (as normally happens during cleaningoperations) the torque at the top of the 3.5″ DP section 300I willexceed the desired torque value of that section 300I, and there will bea higher than normal risk of torsional damage (twisting off) at theinter-connection between the two sections 300 u, 300I of DP.

In the FIG. 9 embodiment, the torque limiting tool 310 is located at amid-way position in the 3.5″ section of DP 300I, and is spaced away fromthe step in the diameters between the upper and lower sections of DP.The torque limiting tool of the FIG. 9 embodiment is set to dislocate ata value less than the maximum capacity of the lower section of 3.5″ DP.Tests performed by the inventors indicate that the FIG. 9 arrangementwith the torque limiting tool located in the lower section 300I, spacedaway from the transition between the two different diameters, is equallyeffective at protecting the entire 3.5″ string 300I against torsionalstresses exceeding its capacity.

This embodiment also has the added advantage that the upper part of the3.5″ DP 300I above the torque limiting device 310 (which is rigidlyconnected to the upper 5″ string) continues to rotate with the uppersection of the 5″ DP 300 u, thereby maintaining pipe movement in theupper part of the 3.5″ section 300I and enhancing the cleaning actionand agitation of the fluids at the liner top L in the same area, evenwhen the torque limiting device has disengaged due to a kick orunexpected spike in the torque on the lower section 300I. This is asurprising advantage, as it resists settling of particulates on theshelf of the liner top L even while the bottom part of the lower section300I has been disengaged and is not rotating with the upper section 300u.

FIG. 10 shows a work string 400 in a similar arrangement having upperand lower parts 400 u and 400I, located in similar diameters of casingand having scraper, brush or other cleaning tools 401. The work string400 is provided with first 410 and second 411 torque limiting deviceslocated in the lower section of DP 400I, and typically arranged todisengage at different thresholds. Typically the upper device 410 can beset to disengage at a higher threshold than the lower device 411.

Optionally more than two torque limiting device can be incorporated intothe string 400, for example 3, 4 or more torque limiting devices can beincorporated into the string 400. Typically the torque limiting devicescan be spaced apart from one another in the string as shown in FIG. 10,but in other embodiments the torque limiting devices can be groupedtogether, in a single cluster having 2 or more torque limiting devices(e.g. 2, 3, 4 or more adjacent devices per cluster) or in 2 or moreclusters that are spaced apart from one another along the string 400.Optionally the torque limiting devices 410, 411 (and others if present)can be set to disengage (and thereby allow relative rotation between thestring above and below each respective torque limiting devices) atdifferent thresholds of torque. Thus the torque limiting devices 410,411 set at different parts of the string 400 can be set to disengage atdifferent torque thresholds, so that as the torque experienced by thestring increases, different section of the string disengage in a safemanner, at different times corresponding to particular threshold torquevalues of the devices 410, 411.

In some modified embodiments of the FIG. 10 arrangement, optionally thetorque limiting devices 410, 411 set at different parts of the string400 can be set to disengage at the same torque threshold, so that oncethat threshold is reached, spaced apart sections of the string disengageat the same time.

Thus, when using more than one torque limiting device, as in the FIG. 10embodiment, the threshold torque limits for each device 410, 411 may beset to different values or to the same or similar values.

In the FIG. 10 embodiment, the devices 410, 411 can be the same ordifferent. For example, in the FIG. 10 embodiment, the first torquelimiting device 410 is typically provided in accordance with the firstembodiment, and is configured to rotationally disengage the two parts ofthe string 400 on either side of the device 410 at a particular torquethreshold and can automatically re-engage the two parts and permit thetransfer of torque across the connection when the torque level dropsback down below the threshold torque value. The second torque limitingdevice 411 can optionally be a different design of a “one-shot” torquelimiting device that is not re-engagable, and in which, after the torquethreshold has been reached and the device 411 has disengaged the twoparts of the string, the device 411 cannot automatically reset in situ,so that when the torque level through the connection across the seconddevice 411 drops back below the threshold torque value, the device 411remains disengaged, preventing the transfer of torque across theconnection, and typically requiring intervention to re-set the one-shotdevice 411. Further variants of this embodiment can have 1, 2, 3, 4 ormore of each kind of one-shot and re-engageable torque limiting device,optionally spaced apart from one another in the string, and each can beset to the same or similar or different torque thresholds. The one-shotdevice(s) can be above or below the re-engagable devices in the string400.

Accordingly, FIGS. 9 and 10 show how it is not necessary to disconnectthe entire 3.5″ string at the transition point and disconnecting aproportion of the string is equally effective at preventing excessivetorque in the 3.5″ DP.

Modifications may be made to the invention herein described withoutdeparting from the scope thereof. For example any number of pistons canbe selected. The profile on the top of each piston and the mating recesscan be varied, for example, a part spherical profile could be used andindeed a ball could be mounted at the end of the piston to achieve this.While disc springs have been described any resilient member capable ofexpansion and contraction may be used such as elastomers, or otherspring types, to bias the pistons.

FIGS. 11-13 of the drawings illustrates a torque limiting devicesuitable for incorporating into a work string, and generally indicatedby reference numeral 510, according to a further embodiment of thepresent invention. The upper end of the device 510 closest to thesurface is shown on the left hand side of the drawing, and the lower endof the device closest to the bottom of the hole is shown on the righthand side of the drawings. The device 510 has an upper portion 512 witha box connection for incorporation into the string, and a lower portion526 having a pin connection, for connecting the device 510 into the workstring. The upper portion comprises an upper collar 512 c and a housing512 h connected via screw threads below a downwardly facing shoulder 512s in the collar 512 c. The housing 512 h has a bore 512 b that receivesa shaft 524 s of the lower portion 526. Below the housing 512 h theupper portion has a spring sub 512 r, and a key housing 513 attachednon-rotatably via screw threads to the spring sub 512 r. The bore 512 bextends coaxially through all of the components of the upper portion512.

The lower portion 526 is radially spaced from the inner wall of the bore512 b by bearings 514. Two races of bearings 514 are shown in theexample described, which typically contain balls, but rollers or otherdesigns of bearings can be used, and more or less than two races can beprovided, e.g. 3, 4, 5 or some other number, in order to centralise theshaft 526 s within the bore 512 b, and to reduce the friction duringrelative rotation of the shaft 526 s within the bore 512 b.

The bearings 514 are typically axially spaced in the device 510 byspacer sleeves extending radially between the inner surface of thehousing 512 h and the outer surface of the shaft 526 s. The spacersleeves are arranged in radially outer and inner concentric pairs, andtypically incorporate seals such as o-rings that seal between the outerspacer sleeves and the inner surface of the bore 512 b, and between theinner spacer sleeves and the outer surface of the shaft 512 s, leaving afluid pathway between radially adjacent sleeves. Optionally resilientdevices such as Belleville spring stacks can be incorporated betweenaxially adjacent pairs of spacer sleeves, thereby applying compressiveforce to the outer and inner sleeves. The number of spring stacks, andthe nature of the resilient device, can be modified within the scope ofthe invention.

The radially outer sleeves are compressed between a downwardly facingshoulder in the housing 512 h and an upwardly facing end of the springsub 512 r that is screwed into the end of the housing 512 h, between thehousing 512 h and the key housing sub 513.

The radially inner sleeves are compressed between the lower end of a capnut 526 c screwed onto the upper end of the shaft 512 s, and an upwardlyfacing shoulder on the outer surface of the shaft 512 s, which can beoptionally provided by the upper end of a vent sleeve 526 v that can bescrewed onto the shaft 512 s.

When assembled with the cap nut 526 c and the spring sub 512 r tightenedup, the outer and inner sleeves are compressed and the spring stackscompress the bearings 514. The spring stacks can be preloaded withdifferent amounts of force, typically around 15,000 pounds, which istypically about 40-60% of their compressive limit, so that axial impactssustained in use below that threshold will not affect the stacks, butabove the threshold, the stacks will typically compress further,protecting the bearings against damage. The threshold can be varied inaccordance with the desired operating parameters.

The vent sub 526 v typically has a radial vent to permit fluid to passbetween the bore 526 b of the shaft 526 s and the outside of the shaft526 s. Fluids flowing through the device can typically pass into theannular area between the shaft 526 s and the housing 512 h at the upperend of the shaft 526 s, and this is beneficial as it lubricates thebearings 514. The seals on the spacer sleeves typically route the fluidflowing down the annular area in to the annulus between the inner andthe outer sleeves, to direct the fluids towards the bearings andrestrict fluid flow elsewhere, thereby reducing erosion. The fluids canflow out of the annular area through the radial vent in the vent sub 526v, back into the bore 526 b.

The lower portion 526 is typically axially constrained within the bore512 b of the housing which is rotationally fixed to the upper portion512 by means of the screw thread. The upper portion 512 and lowerportion 526 are rotationally connected together by a torque limitingconnection 515 operating between them. The torque limiting connection isprovided by a frangible element housed in the key housing shown best inFIG. 12.

The key housing 513 is rotationally connected to the upper portion 512by the screw thread connection. The bore of the key housing 513 receivesthe shaft 526 s of the lower portion 526. Rotation of the lower portion526 within the key housing 513 is restricted by a key 535 extendingbetween the radially opposing inner surface of the key housing 513,which is rotationally fixed to the upper part of the string, and theouter surface of the shaft 526 s, which is rotationally fixed to thelower part of the string and is rotatable relative to the top sub athigh torque loads that dislocate the connection 515.

Optionally the connection 515 comprises a key 535 which in thisembodiment can comprise a flat plate which can be held in opposinggrooves in the inner surface of the key housing 513 and the outersurface of the shaft 526 s. Optionally the plate can be attached to oneof the components, or both of them, or neither. The plate can typicallybe composed of brass or some other metal that has a lower capacity toresist torsional forces than the shaft or the key housing, so that itsacrificially breaks in preference to the other components. The lowercapacity for torsional resistance can be engineered as a result ofweaker material, weakened fracture points in the plate, or a combinationof these and other factors. The threshold at which the plate breaks canbe determined by selecting the material, length, thickness, structure,and/or other characteristics of the plate. Also, although one plate isshown in the sectional view of FIG. 12, more than one plate could bearranged either axially or circumferentially spaced away from the plateshown in the drawings, thereby increasing the threshold of the failureof the connection 515. The key 535 can comprise a pin, plate, or someother component.

Once the threshold is reached and the key is damaged, the lower portionis free to rotate within the bore of the upper portion. The connection515 is made up by withdrawing the tool to surface and installing anotherintact key 535.

The torque limiting device 510 of FIGS. 11-13 can typically beincorporated into a cleaning or other work string, optionally having acirculating device above the torque limiting device 510 and a cleaningtool optionally below it. Typically the torque limiting device 510 couldbe positioned at the lower end of an upper string of drill pipe having arelatively high resistance to torque, typically higher than the cleaningstring below the device 510. If the torque values applied through theconnection between the upper string and the lower string were tomomentarily exceed the limits of the lower string then the torquelimiting device would disengage to avoid transferring the high torquevalues above it to the weaker cleaning string below it. However, it ispossible to place the device 510 at another location in the string asdescribed in earlier embodiments.

The string can be made up as described in earlier embodiments, which canoptionally incorporate other torque limiting device as described herein.According the string can optionally contain only the torque limitingdevice 510, or only another torque limiting device described here, orone of each, or more than one of each. Different devices can be set todisconnect at different thresholds in the string.

During a typical cleaning operation the torque limiting device 510typically operates passively to de-couple the upper and lower stringsfrom one another when the torque passing through the tool exceeds thepreset value thus disengaging the part of the string below the device510. Typically the limit is set to de-couple the string at a thresholdtorque value below the capacity of the lower string so that no torque istransmitted to the lower string above the threshold torque value that itcan bear.

During cleaning operations the total torque reading for the completestring can be monitored from the surface and plotted to identify suddendrops in torque which would indicate likely de-coupling of the tool andrelative rotation of the upper and lower strings.

At higher torque values, the torque will eventually increase to athreshold that disengages the connection device 515. Below thethreshold, the upper and lower portions 512, 526 will rotate togetherand transmit torque through the tool as described above. However, oncethe torque transmitted through the connection device 515 rises above thethreshold torque value, the key 535 is broken by the torque transmittedthrough the connection 515, and no torque is transmitted across thedevice 510.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A torque limiting apparatus for a work string of an oil or gas well,the torque limiting device comprising a body having an axial boretherethrough for fluid flow; the body comprising a first member and asecond member interconnected to be relatively rotatable; the first andsecond members being interconnected by a torque limiting device adaptedto transfer torque between the first and second members up to athreshold torque value, thereby rotationally coupling the first andsecond members together at torque levels below the threshold torquevalue, and wherein the torque limiting device is adapted to disengagethe connection between the first and second members when torque appliedacross the body is greater than the threshold torque value, wherebyafter disengagement of the torque limiting member at least a portion ofthe first member can rotate relative to at least a portion of the secondmember, wherein the torque limiting device comprises at least onefrangible elongate plate restrained between the first and second membersto transfer torque between the first and second member below thethreshold torque value and configured to break when the torquetransferred through the elongate plate exceeds the threshold torquevalue.
 2. A torque limiting apparatus as claimed in claim 1, having atleast one bearing device incorporating a bearing configured to reducefriction during relative rotation of the first and second members.
 3. Atorque limiting apparatus as claimed in claim 2, wherein the apparatusincorporates fluid pathways to guide a proportion of the fluid conductedthrough the torque limiting device through the fluid pathways tolubricate the bearing using the fluids passing through the bore of theapparatus.
 4. A torque limiting apparatus as claimed in claim 2, whereinthe bearing device incorporates a resilient member configured to apply acompressive force between the first and second member to preload thebearing device.
 5. A torque limiting apparatus as claimed in claim 1,having more than one elongate frangible element restrained between thefirst and second members.
 6. A torque limiting apparatus as claimed inclaim 1, wherein said at least one frangible elongate plate isrestrained parallel to the axis of rotation of the first and secondmembers.
 7. A torque limiting apparatus as claimed in claim 1, whereinsaid at least one frangible elongate plate is generally rectangular,having at least two long sides and two short sides, wherein the longsides are opposite one another, and are relatively longer than the atleast two shorter sides, and wherein the plate is engaged by the firstand second members along the long sides of the plate.
 8. A torquelimiting apparatus as claimed in claim 7, wherein the long sides of theplate are retained by slots formed on opposing surfaces of the first andsecond members.
 9. A work string assembly for use in an oil or gas well,the work string having a first tubular and a second tubular, and atorque limiting apparatus as claimed in claim 1 connected in the string.10. A work string assembly as claimed in claim 9, having a selectivelyactuable circulation port provided in the string above the torquelimiting device.
 11. A work string assembly as claimed in claim 9,wherein the first tubular has a different diameter than the secondtubular, and the string incorporates a crossover sub at a connectionbetween the first and second tubulars.
 12. A work string assembly asclaimed in claim 9, incorporating at least one well tool selected fromthe group consisting of: at least one cleaning tool, at least onefishing tool, and at least one jarring tool.
 13. A work string assemblyas claimed in claim 9, wherein one of the tubular members has a lowercapacity for resistance to torsion than the other.
 14. A work stringassembly as claimed in claim 9, having first and second torque limitingdevices connected within the work string and spaced apart from oneanother along the work string.
 15. A work string assembly as claimed inclaim 14, wherein at least one of the first and second torque limitingdevices is connected within the second tubular member such that an upperportion of the second tubular member is located above the torquelimiting device, and a lower portion of the second tubular member islocated below the torque limiting device, whereby upon disengagement ofthe torque limiting device, the upper portion of the second tubularmember is rotationally coupled to the string above it and the lowerportion is rotationally de-coupled from the string above it.
 16. A workstring assembly as claimed in claim 14, wherein one of the first andsecond torque limiting devices is set to re-engage and permit thetransfer of torque across the device when the torque level drops backdown below the preset load, and wherein the other of the first andsecond torque limiting devices cannot reset after the preset load hasbeen reached and the device has disengaged, so that when the torquelevel through the connection drops back below the preset load, saidother device remains disengaged.
 17. A work string assembly as claimedin claims 14, wherein the first and second torque limiting devicesdisengage at different preset loads.
 18. A work string assembly asclaimed in claim 14, wherein at least two torque limiting devices areset to disengage at the same threshold torque value, so that once thatthreshold torque value is reached, spaced apart sections of the stringdisengage at the same time.
 19. A method of operating a work string in awellbore in an oil or gas well, where the wellbore has an upper portionand a lower portion, the method comprising inserting the work stringassembly into the wellbore being treated, the work string assemblyhaving upper and lower portions of the work string, and at least onetorque limiting apparatus comprising a body having an axial boretherethrough for fluid flow; the body comprising a first member and asecond member interconnected to be relatively rotatable; the first andsecond members being interconnected by a torque limiting device adaptedto transfer torque between the first and second members up to athreshold torque value, thereby rotationally coupling the first andsecond members together at torque levels below the threshold torquevalue, and wherein the torque limiting device is adapted to disengagethe connection between the first and second members when torque appliedacross the body is greater than the threshold torque value, wherebyafter disengagement of the torque limiting member at least a portion ofthe first member can rotate relative to at least a portion of the secondmember, wherein the torque limiting device comprises at least onefrangible elongate plate restrained between the first and second membersto transfer torque between the first and second member below thethreshold torque value and configured to break when the torquetransferred through the elongate plate exceeds the threshold torquevalue; and wherein the method includes the step of injecting fluidsthrough the work string assembly to displace fluids in the wellbore andallowing relative movement of the upper and lower portions of the workstring assembly when the torque between the upper and lower portions ofthe work string assembly exceeds the threshold torque value and breaksthe frangible elongate plate.
 20. A method as claimed in claim 19,wherein the upper portion of the wellbore has a larger inner diameterthan the lower portion.
 21. A method as claimed in claim 19, wherein themethod includes landing the torque limiting apparatus in the region ofthe connection between the upper and lower portions of the wellborebeing treated.
 22. A method as claimed in claim 19, wherein the workstring assembly incorporates cleaning tools below the torque dislocationdevice, wherein the well bore is cased, and wherein the method includesbrushing or scraping the inner surface of the casing to remove debrisfrom the wellbore prior to displacing fluids past the connection in thework string assembly.
 23. A method as claimed in claim 19, wherein thework string assembly incorporates a circulation port above the torquedislocation device, and wherein the method includes the step ofcirculating fluid through the circulation port after debris has beenwashed up the annulus between the wellbore and the work string assemblyand has passed the circulation port.
 24. A method as claimed in claim19, having first and second torque limiting devices connected within thework string assembly and spaced apart along the string, wherein at leastone of the first and second torque limiting devices is connected withinthe second tubular member such that an upper portion of the secondtubular member is located above at least one torque limiting device, anda lower portion of the second tubular member is located below the atleast one torque limiting device, whereby upon disengagement of the saidat least one torque limiting device, the upper portion of the secondtubular member is rotationally coupled to the string above it and thelower portion is rotationally de-coupled from the string above it, andwherein the method includes the step of rotating the upper portion ofthe second tubular member with the upper portion of the string afterdisengagement of the torque limiting apparatus.
 25. A method as claimedin claim 24, wherein the first and second torque limiting devices areconfigured to disengage the connection between the first and secondmembers at different preset loads.
 26. A method of treating a linerportion located in a casing in a well bore, comprising the steps: (a)locating a torque limiting device in a tool string; (b) running the toolstring in the well while rotating the string and the torque limitingdevice; (c) circulating a fluid through the string and up an annulusbetween the string and the liner to the liner portion to be cleaned (d)allowing the torque limiting device to disengage under the appliedtorque in the liner; and (e) passing fluid through the casing at theliner portion.
 27. A method as claimed in claim 26, wherein the workstring includes a circulation tool located in the region of the liner tobe cleaned, the circulation tool having a port passing through the wallof the work string, and the method includes the step of opening the portin the circulation tool to pass fluids from the bore of the work stringthrough the port and into the annulus between the work string and theliner.
 28. A method as claimed in claim 24, including the step ofautomatically re-engaging at least one torque limiting apparatus in thework string assembly after the torque drops below a threshold torquevalue, to allow transmission of torque across the at least one torquelimiting apparatus.
 29. A method as claimed in claim 24, wherein atleast one torque limiting apparatus in the work string assembly isarranged to disengage at a preset load, and then stay in that disengagedconfiguration, without re-engaging after the torque drops below thepreset load.
 30. A torque limiting apparatus for a work string of an oilor gas well, the torque limiting device comprising a body having anaxial bore therethrough for fluid flow; the body comprising a firstmember and a second member interconnected to be relatively rotatable;the first and second members being interconnected by a torque limitingdevice adapted to transfer torque between the first and second membersup to a threshold torque value, thereby rotationally coupling the firstand second members together at torque levels below the threshold torquevalue, and wherein the torque limiting device is adapted to disengagethe connection between the first and second members when torque appliedacross the body is greater than the threshold torque value, wherebyafter disengagement of the torque limiting member at least a portion ofthe first member can rotate relative to at least a portion of the secondmember.
 31. A torque limiting apparatus as claimed in claim 30, whereinthe first member has a first surface having at least one recessthereupon; the second member has at least one movable protrusion, eachprotrusion being biased by a resilient device and being movably arrangedin the second member such that the resilient device biases theprotrusion into the recess on the first member to cause the first andsecond members to rotate together; and wherein the recess and theprotrusion have slidably engageable surfaces such that when a torqueapplied across the body is greater than a preset load of the resilientdevice, the protrusion will disengage from the recess and at least aportion of the first member will rotate relative to at least a portionof the second member.
 32. A torque limiting apparatus as claimed inclaim 31, wherein the first and second members have a common axis, andwherein the protrusion(s) can move axially parallel to the axis toengage and disengage with the recess.
 33. A torque limiting apparatus asclaimed in claim 31, having more than one protrusion and recess, andwherein each protrusion has a cam surface adapted to engage anddisengage axially from a recess when the first and second membersrelative to each other.
 34. A work string assembly for use in an oil orgas well, comprising a work string having a first tubular and a secondtubular, and at least first and second torque limiting devices connectedwithin the work string, wherein the first and second torque limitingdevices are spaced axially along the string, and wherein each of thefirst and second torque limiting devices comprises a body having anaxial bore therethrough for fluid flow; the body comprising a firstmember and a second member interconnected and adapted to transfer torquebetween the first and second members up to a threshold torque value,thereby rotationally coupling the first and second members together attorque levels below the threshold torque value, and wherein the torquelimiting device is adapted to disengage the connection between the firstand second members when torque applied across the body exceeds thethreshold torque value, whereby after disengagement of the torquelimiting member at least a portion of the first member can rotaterelative to at least a portion of the second member.
 35. A method ofoperating a work string in a wellbore in an oil or gas well, where thewellbore has an upper portion and a lower portion, the method comprisinginserting the work string assembly into the wellbore being treated, thework string assembly having upper and lower portions of the work string,and at least two torque limiting devices spaced apart along the stringand having first and second members connected together and configured totransfer torque between the first and second member below a thresholdtorque value, and configured to disengage from one another when torquetransferred across the connection exceeds the threshold torque value,whereby after disengagement of each torque limiting device at least aportion of the first member can rotate relative to at least a portion ofthe second member.
 36. An assembly for use in a wellbore, comprising: aworkstring; an upper torque limiting device comprising: a first memberconnected to a second member such that torque applied to the firstmember below a first threshold torque value is transferred to the secondmember, and wherein torque applied to the first member above the firstthreshold torque value is not transferred to the second member; a lowertorque limiting device comprising: a third member connected to a fourthmember such that torque applied to the third member below a secondthreshold torque value is transferred to the fourth member, and whereintorque applied to the third member above the second threshold torquevalue is not transferred to the fourth member.
 37. The assembly of claim36, wherein the first threshold torque value is substantially equal tothe second threshold torque value.
 38. The assembly of claim 36, whereinthe first threshold torque value is greater than the second thresholdtorque value.
 39. The assembly of claim 36, wherein at least one of theupper and lower torque limiting devices comprises the torque limitingdevice of claim 1.